Clean the "CentOS" branch, so that it should work for all
CentOS/RHEL variants (7.x). Make sure to install the
required repositories also in the non-Manila case.
Fedora already ships Ceph packages so does not need additional
repositories.
Do not try to install ceph-radosgw twice.
Finally, explicitly fail for other distributions (not supported
de-facto even in the current code).
Change-Id: I83b1fffad405bc122bc55ef1173f5239f41d2b36